This invention relates to coextrusion of sheet and film product, made especially of thermoplastic compositions such as synthetic resins.
A rotatable distribution pin located proximate to the place of stream convergence, is illustrated by U.S. Pat. No. 4,789,513 to Cloeren. The replaceable distribution pin is provided with the grooved profile shown in FIG. 9 thereof.
The use of an adjustable gap control member for controlling flow from a melt chamber spaced above another melt chamber, to the other melt chamber, is exemplified by U.S. Pat. No. 3,877,857 to Melead. A beveled face of the gap control member cooperates with a wall to form the gap.
Forming a composite stream in a feedblock is illustrated by U.S. Pat. No. 4,784,815 to Cloeren and Wernery, and U.S. Pat. No. 4,695,236 to Predohl et al. Flow pressure of one stream is utilized to displace a portion of another stream. In an embodiment of the Cloeren et al device, stream convergence is effected prior in time to the flow pressure displacement.
In the Predohl et al patent, a rectangular adjusting bar is used to meter side channel flow into a main flow channel. The adjusting bar has a beveled portion located downstream of an input slot of the side channel. The beveled portion compensates flow across the width of the main flow channel. A portion of the adjusting bar may be provided with a concave or convex or sinusoidal profile.
However, a difficulty exists when converging streams of dissimilar rheological properties, that is, for instance, of dissimilar viscosities and elasticities. For example, when a melt stream has a high resistance to flow relative to a melt stream with which it is to be converged, it may be advantageous to provide a heavy edge flow of the high resistance melt stream prior to the convergence by use of a suitably contoured, stream-contacting surface. Similarly, it may be advantageous in other situations to provide a heavy center flow of one of the melt streams prior to the convergence. A drawback of the prior art device just described, is that to handle both possibilities, it would be necessary to replace one specially contoured, stream-contacting surface with a surface of very different geometrical configuration.
Accordingly, there remains a need for an improved extrusion apparatus that is advantageous for converging streams of dissimilar rheological properties. Beneficially, such an apparatus would be useful for the convergence of streams of a variety of rheological properties, without the necessity of replacing a specially contoured, stream-contacting surface.